In the field of infrared detection and imaging, several types of sensors or detectors are known. In each case, the purpose of the detector is to receive a non-visible photon in the medium wavelength infrared (MWIR), long wavelength infrared (LWIR), or very long wavelength infrared (VLWIR), and to convert that photon to either an electrical signal, which can be sensed, or to a visible wavelength photon, which can be perceived directly. In many applications, the infrared detectors are formed in arrays, often called focal plane arrays, which have a plurality of sensing elements so that information for many picture elements can be collected at one time. Such infrared detectors have been used in military and civilian night vision cameras for a variety of purposes.
Although mercury cadmium telluride (HgCdTe or MCT) photovoltic detectors have been used with high performance, recently quantum well infrared photodetector (QWIP) structures have been increasingly used in order to avoid the manufacturing difficulties that are associated with MCT. QWIP detectors are based on a layered structure generally containing a plurality of gallium arsenide (GaAs) layers alternating with a plurality of aluminum gallium arsenide (AlGaAs) layers. Although GaAs/AlGaAs has frequently been used in QWIPs, other materials can also be used. The GaAs/AlGaAs QUIP structure has significant producibility and uniformity advantages over photovoltic MCT devices.
Various designs for QWIP detectors are known. One article presenting such designs is "Quantum-Well Infrared Photodetectors," J. Appl. Phys. 74(8), Oct. 15, 1993, by B. F. Levine. FIG. 1(a), which will be described next, illustrates one such prior art QWIP.
Referring to FIG. 1(a), a prior art QWIP 10 comprises a GaAs substrate layer 30; an aluminum arsenide (AlAs) reflector layer 32, which is deposited on the GaAs substrate layer 30; an N.sup.+ doped GaAs contact 34; a multiple quantum well (MQW) structure 36; and a reflective metalized diffraction grating 38. In this structure, the GaAs substrate 30 faces a scene to be imaged and receives photons which may have been collected and focused by an optical system (not shown). An incident photon 20 enters the GaAs substrate 30 from above and passes downwardly through the GaAs substrate 30, which is transparent in the infrared region, and enters into the MQW region 36 below.
The purpose of the MQW structure 36 is to absorb the photon 20 and to generate a current flow indicative of the absorbed incident infrared radiation. The MQW structure is shown in more detail in FIG. 1b. As shown in FIG. 1b, the MQW structure typically comprises a series of layers of Group III-V semiconductors. A QWIP can comprise, for example, alternating layers of GaAs quantum well layers and AlGaAs barrier layers, and may comprise approximately 50 layers of each. Note, in FIG. 1b, the GaAs quantum well layers and AlGaAs barrier layers are not individually numbered. Often, the GaAs quantum well layers are doped with an n.sup.- dopant, such as silicon, to provide electrons in the ground states of the wells for intersubband detection. When a bias voltage is applied across the MQW structure 36, electrons that are excited by the incident radiation are raised from the ground state energy level in the quantum well into the conduction band above the barrier heights, and can thus flow through the device.
Referring back to FIG. 1(a), the absorption of an incident photon 20 will be described further. In order for an incident photon to excite an electron in a quantum well, the electric field vector of the photon must be perpendicular to the barrier walls. Because the electric field vector for a light beam is normal to its direction of propagation, the direction of the electric field vector is parallel to the barrier walls for a light beam which is incident normal to the detector plane. Since such a light beam has no component of the electric field vector perpendicular to the barrier walls, no energy will be absorbed. To resolve this problem, a reflective metalized diffraction grating 38 is added to the MQW structure on the side of the QWIP which is opposite to the incident photon 20. The reflective diffraction grating 38 reflects and diffracts the incident normal photon 20 within the MQW structure 36. Reflected zeroth order diffraction mode radiation, indicated by arrow 24, is lost by the QWIP. However, higher order diffraction mode radiation, shown by arrows 26, has an electric field vector with a component perpendicular to the MQW barrier walls and can be absorbed. Thus, the reflective diffraction grating 38 substantially improves the absorption efficiency of the QWIP.
A second prior art QWIP 40 is shown in FIG. 2. Like the first prior art QWIP, the second prior art QWIP 40 comprises a multi-layer structure including a thin GaAs substrate 42, an N.sup.+ doped GaAs contact 44, a multiple quantum well (MQW) structure 46, and a reflective metalized diffraction grating 48. The MQW structure 46 of the second prior art QWIP 40 is like the MQW structure 36 of the first prior art QWIP 10, and typically comprises a series of layers of Group III-V semiconductors. The reflective metalized diffraction grating 48 enhances absorption of the incident radiation. As previously described, a reflected zeroth order diffraction mode radiation, indicated by arrow 24, is lost by the QWIP. However, higher order diffraction mode radiation, shown by arrows 26, has an electric field vector with a component perpendicular to the MQW barrier walls and can be absorbed.
In either of the prior art detectors, QWIP 10 and QWIP 40, the quantum well layer and barrier layer can be optimized for LWIR detection. As shown in FIG. 3, for LWIR absorption, each MQW 36 and 46 can comprise approximately 25-50 pairs of layers, each pair comprising a layer of GaAs and a layer of Al.sub.x Ga.sub.1-x As. Each GaAs well 50 has a thickness in the range of about 35 to about 50 Angstroms, and is doped n-type with a doping density of approximately N.sub.D .about.(0.2-1.5).times.10.sup.18 cm.sup.-3. Each AlGaAs barrier layer 54 has a thickness in the range of about 300 to about 500 Angstroms, and is undoped Al.sub.x Ga.sub.1-x As. For LWIR applications, x is typically in the range of about 0.26 to about 0.29. The MQW outer layers include contact layers 56 on either side of the GaAs/Al.sub.x Ga.sub.1-x As sandwich. Each contact layer 56 comprises an N.sup.+ GaAs layer, which is highly doped n-type N.sub.D .about.2.times.10.sup.18 cm.sup.-3. Typically, each contact layer 56 has a thickness in the range of about 0.5 to about 1.0 .mu.m. The MQW structure is epitaxially grown on a lattice matched GaAs substrate.
In each of the prior art QWIPs 10 and 40 described above, an incident photon energizes an electron in a quantum well and, under the influence of an applied bias, contributes to a current flow through the device. An indication of the magnitude of the infrared radiation on the QUIP can be found by measuring the thus induced photocurrent.
Also known in the prior art is the enhanced quantum well infrared photodetector (EQWIP), as described in U.S. Pat. No. 5,539,206, issued to Schimert, the entirety of which is incorporated herein by reference. FIGS. 4 and 5 illustrate the EQWIP. Referring to FIG. 4, a focal plane array using the EQWIP 60, comprises a multi-layer structure wherein a top MQW layer 61 is bonded via a series of indium bump bonds 62 to a readout integrated circuit (ROIC) 63. The top MQW layer 61 has a series of etched wells 64 which extend from the top surface down through a portion of the MQW layer. Each indium bump bond 62 electrically connects one pixel in the top MQW layer 61 to the ROIC 63 so that the signal from that pixel can be measured.
FIG. 5 illustrates a partial cross-section of the EQWIP 60 along the line 5--5 in FIG. 4. Referring to FIG. 5, the EQWIP 60 has a plurality of MQW structures 65A-65E formed above an N GaAs contact layer 66. Below the N.sup.+ GaAs contact layer 66 is an Au reflector 67, with an electrical contact 68 disposed between a portion of the N.sup.+ GaAs contact layer 66 and the Au reflector 67. The thickness of the Au reflector 67 is approximately 2,000 Angstroms, and the reflector 67 is formed on the side of contact layer 60 opposite from the MQW structures 65A-65E. Each of the MQW structures 65A-65E comprises a multilayered MQW portion 69, similar to that shown in FIG. 3, and a top N GaAs contact layer 56.
It should be noted that FIG. 5 illustrates one pixel element within the array of pixel elements in the focal plane array detector shown in FIG. 4. FIG. 5 is dimensioned in accordance with a detector tuned for optimum response to LWIR radiation having a wavelength in the 8-10 .mu.m range. In this structure, the plurality of spaced apart vertically elongated MQW structures 65A-65E form a diffraction grating to diffract incident photons which may be incident normal to the detector surface so that they can be absorbed. Additionally, the multilayered MQW portion 69, of the elongated MQW structures 65A-65E, absorbs those incident photons that have a component of their electrical field vector perpendicular to the plane of the quantum well layers. Thus, the MQW structures 65A-65E of the EQWIP 60, perform the functions of the MQW infrared absorbing structure of the conventional QWIP 36, as well as the metalized diffraction grating 38 necessary to promote absorption of incident photons.
As with the prior art QWIPs 10 and 40 which were described above, in the EQWIP 60 an incident photon energizes an electron in a quantum well of the multilayered MQW portion 69 and, under the influence of an applied bias, the excited electron contributes to a current flow through the device. An indication of the magnitude of the infrared radiation on the QWIP can be found by measuring the thus induced photocurrent.
In addition to infrared detectors which generate a photocurrent in response to an incident photon, also known in the prior art are visible and infrared sensitive photocathodes. In general, a photocathode comprises a multilayered structure including a transparent substrate; an absorption layer, which absorbs an incident photon and which, upon absorption, converts the photon to an electron-hole pair; and an emission layer, having a negative electron affinity at the back side to emit the freed electron into a vacuum. Thus, an incident photon excites an electron in the absorption layer so that the electron is ejected from the structure into a vacuum. In detection systems using photocathodes, the ejected electron is often accelerated over some distance in the vacuum under the influence of an applied electric field to increase the electron's energy, and the thus accelerated electron is subsequently detected with a device such as a charge-coupled device (CCD) or microchannel plate with fluorescent screen.
FIG. 6 illustrates a semiconductor photocathode generally. Referring to FIG. 6, the semiconductor photocathode 70, comprises: a transparent P+ type semiconductor substrate 72, which receives an incident photon 73; an absorption layer 74, formed of P-type semiconductor material for converting a photon 73 to an electron-hole pair; a transport layer 75, which transports electrons which are liberated in the absorption layer 74 to subsequent layers; an electrode 76, for applying a bias voltage V between the substrate 72 and the electrode 76 so that an electric field can be created across the layers 72-76; and a negative electron affinity surface 77, which allows the transported electrons 78 to easily escape the material and be emitted into the vacuum 79.
Also known in the prior art is an infrared photocathode detector which uses an InAs/Ga.sub.w In.sub.y Al.sub.1-y-w As type II superlattice for an infrared absorbing layer, where w+y&lt;1 and where w and y define the mole fraction of Ga, In and Al. In the type II superlattice, coupling between closely spaced material layers creates an effective direct bandgap due to allowed conduction and valence band states. Unlike the QWIP, the superlattice absorber can absorb light from any angle relative to the semiconductor surface.
As illustrated by the foregoing discussion, a plurality of types of infrared detector structures are known. In the general field of detector development, the current objectives are to increase the performance of detectors, decrease difficulties in manufacturing, and reduce the cost of producing detectors. In view of the desirable manufacturing qualities of the QWIP and EQWIP over MCT, infrared detectors based on a QWIP or EQWIP structure have potential cost advantages.